The present invention relates generally to modulation techniques, and, more particularly, to a system, and associated method, for synthesizing a modulated signal.
A radio communication system permits transmission of information between a transmitter and a receiver. The transmitter and the receiver are interconnected by a radio-frequency channel to permit transmission of information therebetween.
By combining the information with a radio-frequency electromagnetic wave of a particular frequency, i.e., modulating the information signal onto a carrier, the resultant, modulated information signal may be transmitted through free space to transmit thereby the information from the transmitter to a receiver. Various modulation techniques (such as, for example, amplitude, frequency, phase, and composite modulation) are known to combine the information signal with an electromagnetic wave.
The total communications capacity in a geographic area is limited by the finite amount of electromagnetic spectrum available. Increasing capacity is most practically achieved by increasing the efficiency of the modulation method used; i.e. by reducing the amount of frequency spectrum required to transmit the information signal. For example, when the information signal is a speech waveform, significant redundancy is present. Some of this redundancy can be removed by using one of a variety of known techniques for encoding the speech waveform with an encoder. The encoded information signal uses less bandwidth.
The output of such an encoder is a discrete binary data stream, whose elements represent various characteristics of the input speech waveform. This binary data stream can be appropriately filtered and modulated onto a carrier at a frequency appropriate for transmission. Independent of modulation method, this binary data stream, because it represents the speech waveform with some of the redundancy removed, requires less bandwidth to transmit, relative to the original speech information signal. The reduced bandwidth permits either more frequency channels for a given allocation of spectrum, or intermittent use of channels in place of continuous use as required for direct speech waveform modulation.
Transmission of the information in discrete form is also advantageous for the reasons that the audio quality of a discrete, encoded signal is superior to a signal generated by conventional continuous wave techniques as the noise components of the transmitted signal may be removed from the signal during reconstruction thereof by the receiver decoder which reconstructs the speech signal from a received estimate of the binary data stream. Still further, scrambling of a signal may be much more easily effectuated as a discrete, encoded signal may be scrambled by the information source and descrambled by an information receiver more easily than a continuous wave analog signal. Additionally, data generated by the vast majority of computer systems is in discrete form, and such discrete information may be more easily transmitted by a discrete encoder rather than by conventional continuous wave techniques. Still further, discrete, encoded signals may be transmitted at various transmission rates (such as, for example, in terms of baud rate--e.g., kilobits per second). Discrete, encoded signals transmitted at higher bit rates, when decoded, recreate a better quality signal. Different levels of service are thereby possible. The radiotelephone system operator, who provides service to users at a price, may permit users to transmit encoded signals at different bit rates at prices corresponding to the bit rates of transmission.
Further efficiency can be achieved by choice of the method of modulating the information signal (of whatever form) onto the carrier. In traditional mobile radiotelephone service, the modulation method is usually frequency modulation (FM). This method, because it passes information only through the phase of the carrier (the amplitude is constant) uses more bandwidth than necessary. Composite modulation, in which information is encoded in both the amplitude and the phase of the carrier signal, uses channel capacity more efficiently.
However, existing composite-modulation communication systems and apparatus therefor are quite complex. Conventional practice is to separate the transmit carrier source (or a carrier intermediate frequency, i.e., IF, source) into sine and cosine components, mixing each component with separate portions of the information signal input, and summing the mixer outputs. The resultant modulated signal, if it is at carrier frequency, can be applied directly to an amplifier for amplification to final transmit power level. If the resultant modulation signal is at a carrier IF, it is further multiplied by a LO oscillating signal to shift it in frequency to the proper carrier frequency, and then amplified. See, for instance, a discussion in the text Introduction to Communication Systems, 2nd Ed, by Ferrel G. Stremmler, ISBN 0-201-07251-3, pages 590-596.
A family type of composite modulation is quadrature amplitude modulation (QAM). In this modulation method, as conventionally applied to a binary information source, the binary data stream is separated into bit pairs. The individual bits of these bit pairs are converted from unipolar to bipolar format, passed through a pair of electric wave filters, and applied to the multiplier pair whose other inputs are the sine and cosine components of the carrier or carrier IF signal. A particular type of QAM is .pi./4-shift DQPSK (for differential quadrature phase shift keying), in which the input data stream is encoded so that the composite modulated carrier shifts in increments of .+-..pi./4 or .+-.3.pi./4 according to the input bit pairs. This modulation method, conventionally implemented, is discussed in Digital Communications, by John G. Proakis, 1st Ed., ISBN 0-07-050927-1, pages 171-178.
Because the process of mixing the information signal with the carrier wave is essentially a multiplication process, such a process, when implemented by existing communication system apparatus, requires complicated circuitry having precise component constraints.
What is needed, therefore, is a system for synthesizing a modulated signal, such as, for example, a discrete encoded, modulated signal, which does not require precise components and complicated circuitry for synthesis thereof.